Dependency Injection

Dependency injection is a term used to describe a separation between the
implementation of an object and the construction of an object it
depends on, and the ability for a container like Resin
to resolve the dependency.

Since the container instantiates and assembles the dependencies,
the code is simpler and the configuration is more flexible.
It's easy to substitute test implementations as the
dependent resources, for example.

Configuration as Assembly Line

The Dependency Injector pattern could also be called
the Assembly pattern because it resembles an assembly
line making cars.

Parts are interchangable components like wheels. The parts might
also be assembled like an engine in a car.

Parts are attached to the Chassis like a car's frame receiving an
engine.

The Registry is holds partially-completed parts like a factory
conveyor belt.

The Assembler provides the Registry and assembles the Chassis and
Parts into a completed resource.

Some important points:

The application code (Chassis and Parts) is independent of the
Assembler.

Parts are interchangeable.

The code needs to select an assembly pattern, e.g. Setter Injection.

Because the Assembler is independent of the code, a project could
change the Assembler from Spring to Resin with no code changes.
So using the Assembler/Dependency Injection pattern reduces
dependencies on the framework. Only the configuration changes
when changing Assemblers, not the code.

While testing, the test case or the harness plays the Assembler
role, simplifying the test suite and ensuring that the code under test
is the production code. A test can create a test implementation of
the Part, e.g. TestMovieFinder, for testing.

In some cases, the application code can provide its own
assemble() method for situations where the container is
incapabile of assembling the components. For example, the
MovieServlet could create an assemble()
method that grabbed the MovieLocator from JNDI.

Code for the Dependency Injection pattern

The only code specific to the setter-based injection pattern
is the addition of a setter method for the dependent resource.
In many application, that setter will already be written, so
no additional code would be required.

Either an interface or a class can be used
for the dependent resource, depending on the application's
architecture. This example uses both: the MovieLister uses a dependent
MovieFinder interface, and the MovieServlet uses the dependent
MovieListener class.

Dependency Injection for Servlets

The Dependency Injection pattern is just as useful for servlet
configuration as it is for resources. This example makes the
MovieLister a parameter of the servlet. The resin-web.xml will configure
the servlet with the appropriate MovieLister

The advantages of using dependency injection for the servlet are
the same as for the resource:

The servlet code becomes simpler.

The servlet is no longer dependent on JNDI.

The servlet is more easily testable by configuring it with test
versions of the MovieListener.

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